1. Field of the Invention
The present invention relates to an optical interferometer. Specifically, the present invention relates to an optical interferometer that measures change in thickness of a to-be-measured object.
2. Description of Related Art
As a device for measuring an irregularity of a to-be-inspected surface, an optical interferometer has been known (for example, Document: JP-A-H08-35883).
FIG. 8 shows a Michelson interferometer 10.
In FIG. 8, light emitted from a light source 11 becomes a parallel light beam by a collimator lens system 12 and the parallel light is transmitted through a half mirror 13 and narrowed by a lens 14 disposed between the half mirror 13 and a workpiece W to be irradiated on the workpiece W. The light irradiated on the workpiece W is reflected by the workpiece W to be object light La. The object light La is reflected by the half mirror 13 to be incident on an interference fringe sensor. Between the lens 14 and the workpiece W, a beam splitter 16 and a reference mirror 17 are disposed. Out of the light having been transmitted through the lens 14, light transmitted through the beam splitter 16 is irradiated on the workpiece W and becomes the object light La, while light reflected by the beam splitter 16 is incident on the reference mirror 17 and reflected by the reference mirror 17 to be reference light Lb.
The reference light Lb is reflected by the beam splitter 16 again to be incident on an interference fringe sensor 15 via the half mirror 13. The interference fringe sensor 15 acquires interference fringes generated in interference between the object light La and the reference light Lb by capturing the interference fringes with a CCD camera (not shown) and the like. By analyzing gaps between the interference fringes, irregularities of a surface of the workpiece W can be obtained.
Where the irregularities of the surface of the workpiece W can be thus obtained using the optical interferometer 10, there are demands for knowing change in thickness of the workpiece W such as change in thickness of a semiconductor wafer. To know change in thickness of the workpiece W, one way is measuring a front surface of the workpiece W and then a rear surface of the workpiece W with the optical interferometer 10 shown in FIG. 8. Specifically, firstly the front surface of the workpiece is measured with the optical interferometer 10 shown in FIG. 8. Then, the workpiece W is turned over and irregularities of the rear surface of the workpiece is measured. By combining profile data of the front surface and the rear surface of the workpiece W, change in thickness of the workpiece W can be known.
By turning the workpiece W over such that a front surface Sa and a rear surface Sb of the workpiece can be measured, each irregularity data of the front surface Sa and the rear surface Sb can be obtained.
However, change in thickness of the workpiece W cannot be obtained if a point of front surface data Da is not matched with a point of rear surface data Db when combining the measured data. Specifically, even when the front surface data Da and the rear surface data Db are obtained as shown in FIG. 9, if coaxially corresponding points of the front surface data Da and the rear surface data Db cannot be identified, change in thickness of the workpiece W cannot be obtained.
FIG. 9 shows a point A1 of the front surface data and a point A2 of the rear surface data, the point A2 being coaxial to the point A1. By matching the front surface data Da of the point A1 with the rear surface data Db of the point A2, data corresponding to the thickness of the workpiece W can be obtained. Thus, by matching the front surface data Da and the rear surface data Db of coaxial points, change in thickness of the workpiece W can be obtained.
However, when a coaxial point corresponding to the point A1 cannot be accurately identified and a point B2 that is displaced off the axis is matched with the point A1, change in thickness of the workpiece W cannot be obtained.
As another way, as shown in FIG. 10, two of the optical interferometer 10 can be respectively disposed on a front surface side and a rear surface side of the workpiece W with the workpiece W interposed. An optical flat (not shown) of which coaxial points of a front surface and a rear surface are identified in advance may be prepared. Before measuring the workpiece W, by measuring both of the front surface and the rear surface of the optical flat in advance, the corresponding points of the front surface side data Da and the rear surface side data Db can be identified.
However, it is difficult to prepare an optical flat of which front and rear surfaces are highly accurately flat without waviness and of which corresponding points of the front and rear surfaces are highly accurately identified.
Accordingly, even if the two optical interferometers 10 are disposed with the workpiece W interposed, it is difficult to obtain change in thickness of the workpiece W by matching the front surface data Da and the rear surface data Db coaxially.
The optical interferometer 10 on the front surface side and the optical interferometer 10 on the rear surface side each have the reference mirror 17.
Note that irregularity change obtained by the interference fringes is difference in irregularity and inclination of the reference mirror 17 and the surface of the workpiece.
Hence, when an irregularity and an inclination of the reference mirror 17 on the front surface side of the optical interferometer 10 differs from those of the reference mirror 17 on the rear surface side of the optical interferometer 10, even if the front surface data Da and the rear surface data Db are combined, a thus obtained value includes difference in irregularity and inclination between the reference mirrors 17 in addition to change in thickness of the workpiece W. In other words, it is not possible to obtain only change in thickness of the workpiece W.
Although it is possible to prepare the optical flat and obtain irregularity and inclination of the reference mirrors 17 in advance, it is difficult to prepare a highly accurate optical flat as described above.
Due to the above-stated problems, the demands for obtaining change in thickness of the workpiece W have not been met.